Dry replenishment of electroless copper solutions

ABSTRACT

This invention is for replenishment of an aqueous electroless copper plating solution and to apparatus useful therefore. The process comprises addition of replenishers in essentially dry form in a manner that avoids triggering of the bath. Typical replenishers that may be added in dry form include one or more of a source of cupric ions such as cupric sulphate, a reducing agent for cupric ions such as paraformaldehyde, a chelating agent for cupric ions, bath stabilizers, brighteners, surfactants and the like. These ingredients may be added singularly or admixed with each other. Essentially dry materials are used for replenishment rather than a solution as in the prior art to prevent volume growth of the plating solution.

United States Patent [1 1 Dutkewych et al. 1 Nov. 6, 1973 [54] DRY REPLENISHMENT OF ELECTROLESS 3,595,684 7/1971 Morico et al. 106/! X COPPER SOLUTIONS 3,649,350 3/1972 Agens 106/1 X [75] Inventors: Oleh B. Dutkewych, Medfield; Primar Examiner Lorenzo 3 Ha es Lebert Hofman westboro both Att0rr r3;Sewa1l P. Bronstein et ali of Mass. I .7 [73] Assignee: Shipley Company, Inc., Newton, [57] ABSTRACT Mass- This invention is for replenishment of an aqueous elec- [22] Filed: Oct. 12, 1971 troless copper plating solution and to apparatus useful therefore. The process comprises addition of replenishl l PP 188,243 ers in essentially dry form in a manner that avoids trig- I gering of the bath. Typical replenishers that may be 521 US. (:1. 106/1, 117/130 E added in y form include one Or more of a Source of 511 1111.01. C23c 3/02 CuPriC ions Such as cupric Sulphate, a reducing agent [58] Field of Search 106/1; 117 130 E, for Cupric ions Such s Paraformaldehyde, a chelating' 1 17/160 A agent for cupric ions, bath stabilizers, brighteners, surl factants and the like. These ingredients may be added [56] References Cited singularly or admixed with each other Essentially dry UNITED STATES PATENTS materials are used for replenishment rather than a solution as in the prior art to prevent volume growth of the 2,819,188 7/1958 Metheny et al. 117/130 E plating Solution 2,938,805 5/1960 Agens 106/1 3,532,519 10/1970 Hirohata et al 106/1 13 Claims, 2 Drawing Figures PATENTED NOV 6 I975 mON EN EN \J mom ||l| ooN wON @ONI. U o vow Q 1 III III II ,v co J I] Y mom k 2N I 2N mom v k &/ /6 v m m: u 5 Now DRY REPLENISHMENT OF ELECTROLESS COPPER SOLUTIONS BACKGROUND OF THE INVENTION INTRODUCTION This invention relates to electroless copper plating and more particularly, to replenishment of depleted electroless plating solutions.

DESCRIPTION OF THE PRIOR ART As is known in the art, electroless copper deposition refers to the chemical deposition on a receptive surface of an adherent copper coating in the absence of an external electric source. Such deposition is useful, for example, in the manufacture of printed electric circuits, as linings for wave-guide cavities, as an initial conductive coating in electro-forming, and fordecoration.

A number of electroless copper deposition processes and solutions have heretofore been known and are disclosed in numerous United States Patents including U.S. Pat. Nos. 2,874,072; 2,938,805; 3,075,856;

3,095,309; 3,222,195; and 3,392,035, all incorporated herein by reference.- 7

Such electrolesscopper deposition solutions typically comprise an aqueous solution of cupric ions, a source of formaldehyde, hydroxide, and a chelating agent to renderthe cupric ions soluble in alkaline solution. Deposition occurs by the reduction of cupric ion to copper by formaldehyde, initiated by the *presenceof a suitable catalytic surface, for example, varioussurfaces of catalyzed plastic as disclosed in US. Pat. No. 3,011,920.

It is known in the art that in use, an electroless copper deposition solution becomes depleted in solution constituents. For example, cupric ion concentration becomes depleted through deposition on a substrate. Formaldehyde becomes depleted as it is used to reduce the cupric ion to metallic copper. The concentration of the chelating agent is somewhat lowered by drag-out on the parts passing through the plating bath. Consequently, with use, as the solution becomes depleted, deposition rate decreases to a point'where the solution is no' longer use'able. I

In order to increase the useful life of an electroless copper deposition solution, it has been the practice in th'eart to replenish the solution with solution constituents at frequent intervals during the plating cycle. This has been accomplished by adding one or more of the solution constituents to the plating solution in the form of a concentrated aqueous solution.

A major difficulty with the prior art procedures for replenishment of an electroless copper deposition solution is that there is an increase in volume of the solution each time it is replenished with a liquid replenisher. This is due to the fact that as the solution becomes depleted of its solution constituents, there is no corre sponding decrease in volume as the only losses in volume occur as a result of evaporation and dragout. For example, in one particular large volume continuous plating operation, there are about five plating cycles per 24 hour period. The plating solution is replenished with aqueous solution constituents after each cycle. Each replenishment results in about 10 percent increase in volume of the plating solution which means that there is about a'50 percent volume increase every 24 hours. As a result, the cost of the plating process is substantially increased and a greater volume of solution must ultimately be dumped. With code regulations preventing dumping of industrial wastes containing c'opper values, this becomes an ever increasing problem.

It is believed that attempts have been made in the prior art to add replenishers in' dry form to avoid volume growth of the plating solution. However, these attempts have been unsuccessful because the most obvious method of adding dry ingredients to a plating solution would be to simply drop the dry powder onto the top of the plating tank. However, this method is not operative because the powder in solution acts as localized areas of high concentration and as nucleating sites for plating resulting in triggering of the bath and loss thereof. Also, particles in the plating solution hitting the work piece to be plated result in defects in the coherency of the metal plate.

STATEMENT OF THE INVENTION The subject invention provides a method of replenishing a stable plating bathwith replenishers that are essentially in dry: form without triggering the bath thereby avoiding loss of solution by volume growth. The method is based upon thediscovery that the replenishers may be added to a stable plating bath if they are kept out of contact with air and are rapidly dispersed and dissolved in solution. This can be accomplished by adding the replenisher below the surface of the solution with agitation to avoid localized areas of high concentration and to facilitate dissolution. Typical replenishers that may be llsed are one or more of a source of cupric ions such as cupric sulphate, a solid source of formaldehyde such as paraformaldehyde, a chelating agent for the cupric ions, caustic and known additives to copper plating baths such as stabilizers, brighteners, surfactants and the like. These ingredients may be added singularly or admixed with each other except that caustic cannot be admixed with formaldehyde unless special precautions are taken as will be discussed in greater detail below; Preferably, atleast the DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 represents a preferred plating apparatus in combination with apparatus for adding dry powders; and

FIG. 2 represents a cross-sectional view of the apparatus of FIG. 1 taken along section line 2-2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For purposes of definition, essentially dry form, as used to describe the replenisher herein, is intended to mean a composition having a solids content of at least percent by weight. Thus, the term contemplates highly concentrated solutions substantially more concentrated that prior art replenishers as well as dry powders. In some instances, moisture content'is desirable as it facilitates molding to tablet or other desired shape.

In this respect, it should be noted that those substances added in smaller concentrations such as replenishment of stabilizer, surfactant, brightener, chelating agent and the like may be in liquid form admixed with dry powders of the cupric salt and the paraformaldehyde as the minor concentration of these constituents will still keep the total solids content within 75 percent.

As noted above, the replenisher composition for the subject invention is in essentially dry form and com prises one or more of a copper salt that is a source of cupric ions in an aqueous electroless copper solution, paraformaldehyde as a source of formaldehyde, a small amount of chelating agent for cupric ions, stabilizers, brighteners, surfactants and the like. The materials contemplated for use in the subject invention are old, exemplified in the above referenced patents and conventionally used in the formulation of electroless copper deposition solutions.

Typical examples of copper salts for purposes of the present invention include cupric sulphate, cupric chloride, cupric nitrate, cupric hydroxide, cupric tartrate and cupric acetate. Typical examples of suitable complexing agents for the cupric ions include Rochelle salts, the sodium salts (mono-, di-, tri-, and tetrasodium salts) of ethylenediaminetetraacetic acid, nitrilotriacetic acid and its alkali metal salts, tartaric acid, modified ethylenediaminetetraacetic acids such as N-hydroxyethylenediaminetriacetate, hydroxyalkyl substituted dialkalenetriamines, sodium salicylate and sodium tartrate. Other complexing agents for cupric ions are disclosed in U. S. Pat. Nos. 2,996,408; 3,075,855 and 2,938,805, all incorporated herein by reference.

Materials known to the art as catalytic poisons to the deposition of electroless metals are frequently used in controlled amounts as stabilizers for the plating solutions. The most widely used group of compounds of this nature are the divalent sulphur-containing compounds, many of which are disclosed in U.S. Pat. No. 3,361,540 and exemplified by inorganic sulfides such as sodium sulfide, organic and inroganic t-hio compounds such as sodium thiocyanate and organic sulphur compounds such as thiourea. Another class of stabilizers are the water soluble cyanide compounds as defined in U.S. Pat. No. 3,310,430 such as sodium cyanide. The third class of stabilizers is disclosed in U.S. Pat. No. 3,457,089 and includes acetylinic alcohols and ethers such as 2-butynel,4-diol. Other stabilizers are known in the art.

The relative proportions of the ingredients comprising the dry replenisher mixture may vary within broad.

limits dependent upon the requirements and use of the electroless copper deposition solutions to which the replenisher is added. In general, the copper salts and paraformaldehyde are added in about the same relative proportions as found in fresh electroless copper deposition solutions, e.g.,.typically paraformaldehyde in an amount capable of yielding from at least 0.5 moles of formaldehyde per mole of copper salt and preferably from 2 to 4 moles per mole of copper. If a chelating agent is used as a constituent of the dry replenisher mixture, its relative proportion in the mixture is substantially less than in the original copper deposition solution as it is not consumed in the plating operation. Typically, the chelating agent may constitute from about 0.01 to 0.50 moles per mole of copper salt.

The dry replenisher may contain substituents in addition to those noted above. For example, additives may be used to facilitate production such as molding agents, of which glycelol is a good example as it is also a chelating agent for the cupric ions. Other additives may include agents to protect against decomposition, sublimation, handling and the like. As noted above, any of the chelating agent, stabilizer, surfactant and brightener, since added in minor proportion, may be in liquid form if blended with the cupric salt, paraformaldehyde or both as the amount of such additive will still result in a replenisher in essentially a dry form having a solids content in excess of percent by weight.

In addition to the above ingredients, hydroxide may require replenishment to maintain necessary alkalinity, the most common source of hydroxide being caustic. However, caustic cannot be mixed with the other ingredients of the dry replenisher and must be added separately. If the source of hydroxide is cupric hydroxide, it may be mixed with the other ingredients though caustic might still be required to ,obtain necessary alkalinity. One method for adding caustic in solid form would be to coat the same with a protective coating which would prevent reaction with formaldehyde which coating should be soluble in the plating solution and not detrimental thereto.

The replenisher should not be added to the electro less copper deposition solution by simply dropping the same onto the surface of the solution as this would cause triggering and interfere with the deposit on a work-piece. It is a discovery of this invention that the replenisher is added to a stabilized bath out of contact with air and in a manner to rapidly disperse powders and to dissolve the same. Thismay be accomplished by adding the replenisher directly to the solution below its surface with solution agitation to facilitate dispersing the dry replenisher in solution and dissolve the same as rapidly as possible.

As noted above, the bath to which the replenisher is added should be substantially stable. Upon depletion of a plating bath, there may also be depletion of stabilizer through drag-out or otherwise. Thus, the bath should be stabilized prior to dry replenishment. This may be accomplished one of several methods. The stabilizer may be blended with the other replenisher ingredients or added final. Alternatively, the situation can be avoided by an initial bath formulation having excess stabilizer provided it does not poison the bath. What cannot be done is the addition of the stabilizer to an un stable bath subsequent to replenishment of other ingredients. Alternatively, the bath may be maintained stable by bubbling air through the same as in known in the art.

A preferred method and apparatus for adding replenisher is illustrated in the drawings, wherein in FIG. 1, there is illustrated an apparatus comprising a plating section to the right of hatched line AA and a replenishment section, greatly enlarged for purposes of illus tration, to the left of hatched line AA. The plating section consists of plating apparatus of a catalytically inactive material such as polyethylene. This section is divided into plating tank 101 and overflow tank 102. Catalytically active parts are plated in plating tank 101 by racking the same and immersing them in the plating tank (parts and racks not shown.). Contact of the catalytically active part with the plating solution will result in plating.

Plating tank 101 is separated from overflow tank 102 by barrier 103 having overflow lip 104, but is in open communication with overflow tank 102 to permit passage of the plating solution therebetween. As the plating solution passes over barrier 103 from plating tank 101 into overflow tank 102, it may pass through filtering means 105 such as a cloth bag where the replenisher may be added. Within overflow tank 102, there may be placed heating coil 106 to maintain desired solution temperature. When replenishment of the solution is not required, valve 107 is closed and valve 108 is opened. Thus, the flow of plating solution is in a closed loop from plating tank 101 to overflow tank 102, out of overflow tank 102 through opening 109 and into plating tank 101 through inlet 110.

When replenishment is desired, valve 108 is left open and valve 107 opened. Solution then flows out of overflow tank 102 through outlet 111, through pipe 201 into replenishment apparatus 200 through outlet 202. Outlet 202 discharges into a venturi type nozzle 203 designed to affect a high speed swirling motion to the solution. The relationship of outlet 202 to nozzle 203 is depicted in FIG. 2 where it can be seen that outlet 202 discharges into the side of nozzle 203 to cause the swirling motion. As the solution passes downward through nozzle 203, it increases in speed due to the decreased diameter of the nozzle. The solution then passes through replenisher powders 204 held in a porous container 205 such asa cloth bag. The swirling motion of the solution causes dissolution of the powder at a relatively constant rate. The replenished relatively concentrated solution then, passes through chamber 206, through outlet 207 and valve 208 which is in the open position and through pipe 209 into the filterbag 105 in overflow tank 102 through outlet 210,

The replenisher powders 204 are added to replenisher apparatus 200 by any convenient means. For example, with reference to the drawing, there is depicted an assembly comprising a hand turn wheel 211 secured with yoke 212. Upon turning the wheel, cap 213 is raised and the entire assembly pivots sideways and downwards on pivot pints 214. With removal of seal 215, there is readily access to the porous container 205.

If it is desired to change or alter container 205, this is readily accomplished by removing clamp 216 from notch'217 which holds container 205 in place.

It should be understood that While the replenisher has been referred to herein as essentially dry, it may be in the form of a molded shape, such as that of a tablet or alternatively packaged in a solution soluble plastic bag such as a bag formed from carboxymethylcellulose. However, using a solution soluble'bag, the concentration of the materials-of the bag might build up in solution to the point where it could contaminate the solution.

The invention will be better understood with reference to the following examples.

EXAMPLE 1 With reference to the apparatus depicted in the drawings, a tank having a capacity of 25 gallons was filled with an electroless copper depositing solution having a formulation as follows:

I .5 pounds r by )6 inch in thickness In addition to the above ingredients, there are added various solution stabilizers and other additives known in the art.

The above solution is maintained at l 25F by recirculating the same from the plating chamber to the overflow tank containing a heating coil and filter bag at a rate of 200 gallons per hour. The solution is used to deposite copper on a catalyzed phenolic substrate in the manufacture of printed circuit boards with a loading of as square foot per gallon of plating solution. After 10 minutes, the solution is depleted in copper, formaldehyde and sodium hydroxide by about. 10 percent of the original amount. After 30 minutes, there is a noticeable decrease in plating rate. A powder mixture is prepared comprising about 5: pound of each of cupric sulphate pentahydrate and paraforrnaldehyde blended together. The blend is placed in a cloth bag in the replenishment apparatus depicted in the drawing. The solution is then recirculated from the overflow tank through the replenishment apparatus at a rate of 200 gallons per hour and back into the filter bag of the overflow tank where it is filtered and recirculated into the plating tank. Within about twenty minutes, plating rate returns to normal and powder constituents in the bag are substantially dissolved. At this point, the flow of fluid through the replenishment apparatus may be discontinued. I g

The following examples of suitable replenisher compositions.

EXAMPLE 2 cupric sulphate pentahydrate 12.5 grams parafonnaldehyde l2.5 grams EXAMPLE 3 cupric sulphate pentahydrate 10 grams paraformaldehyde l0 grams pentahydroxypropyldiethylene I triamine 5 grams EXAMPLE 4 cupric sulphate pentahydrate 10 grams paraformaldehyde 10 grams ethylencdiaminetetraacetic acid 4 grams tetrahydroxypropylethylenediamine l gram EXAMPLE 5 cupric acetate 10 grams paraformaldehyde 10 grams mixed sodium-potassium tartrate 5 grams Each of the above formulations may be molded into tablets measuring approximately 2 inches in diameter if desired.

We claim: I g

1. In a method for replenishing an electroless copper plating solution comprising cupric ions, a source of formaldehyde and a chelating agent to maintain said cupric ions in solution with at least one or more replenisher constituents in essentially dry form, said replenisher constituents being selected from a group of cupric salts, formaldehyde or a source of formaldehyde, chelating agents, stabilizers and mixtures thereof, the improvements comprising maintaining said solution stable, adding and substantially dissolving said replenisher constituents beneath the level of said plating solution while maintaining said essentially dry replenisher constituents substantially out of contact with air and the surface of the plating solution and avoiding localized areas of high concentration of the replenisher constituents by rapidly dispersing the same in said plating solution.

2. The method of claim 1 where the solution is maintained stable by replenishment with a stabilizer prior to or during replenishment with at least the cupric salt.

3. The method of claim 1 wherein the solution is maintained stable by bubbling air through the plating solution.

4. The method of claim 1 where the solution is maintained stable by use of an excess of a non-poisoning stabilizing agent in the original solution make-up.

5. The method of claim 1 where the localized areas of high concentration of replenisher constituents are avoided and rapid dissolution of constituents is accomplished with solution aggitation.

6. The method of claim 1 where the replenisher constituents are added directly to the plating solution beneath the level of the plating solution.

7. The method of claim 1 where said replenisher constituents are added and substantially dissolved beneath the level of the plating solution by circulating a minor portion of the plating solution external to the major portion of said solution, through a mass of said replenisher constituents to substantially dissolve the same whereby the replenisher constituents are maintained substantially out of contact with the surface of the plating solution, and recirculating said minor portion of solution enriched in replenisher constituents back to the major portion of said solution.

8. The method of claim 7 where said minor portion of solution is passed through replenisher constituents.

9. The method of claim 7 where said minor portion of solution has its speed increased as it passes through said replenisher constituents by imparting a swirling motion to said minor portion of solution.

10. The method of claim 7 where said replenisher constituents comprise at least 2 moles of paraformaldehyde per mole of cupric salt.

11. The method of claim 10 where said replenisher constituents also include a minor amount of a stabilize ing agent for the plating solution.

12. The method of claim 10 where said replenisher constituents also include a minor amount of chelating agent.

13. The method of claim 7 where said replenisher constituents are in essentially dry powder form. 

2. The method of claim 1 where the solution is maintained stable by replenishment with a stabilizer prior to or during replenishment with at least the cupric salt.
 3. The method of claim 1 wherein the solution is maintained stable by bubbling air through the plating solution.
 4. The method of claim 1 where the solution is maintained stable by use of an excess of a non-poisoning stabilizing agent in the original solution make-up.
 5. The method of claim 1 where the localized areas of high concentration of replenisher constituents are avoided and rapid dissolution of constituents is accomplished with solution aggitation.
 6. The method of claim 1 where the replenisher constituents are added directly to the plating solution beneath the level of the plating solution.
 7. The method of claim 1 where said replenisher constituents are added and substantially dissolved beneath the level of the plating solution by circulating a minor portion of the plating solution external to the major portion of said solution, through a mass of said replenisher constituents to substantially dissolve the same whereby the replenisher constituents are maintained substantially out of contact with the surface of the plating solution, and recirculating said minor portion of solution enriched in replenisher constituents back to the major portion of said solution.
 8. The method of claim 7 where said minor portion of solution is passed through replenisher constituents.
 9. The method of claim 7 where said minor portion of solution has its speed increased as it passes through said replenisher constituents by imparting a swirling motion to said minor portion of solution.
 10. The method of claim 7 where said replenisher constituents comprise at least 2 moles of paraformaldehyde per mole of cupric salt.
 11. The method of claim 10 where said replenisher constituents also include a minor amount of a stabilizing agent for the plating solution.
 12. The method of claim 10 where said replenisher constituents also include a minor amount of chelating agent.
 13. The method of claim 7 where said replenisher constituents are in essentially dry powder form. 